nothing concrete in mind, but if she's a nurse, i assume she has her life somewhat together? it could've been something like they started off just having fun and intending to keep things light, but developed stronger feelings before they really figured out what they were doing. dan's basically in an entry level job and has no desire to move up or figure out what he actually wants to do in life, which i could see definitely being frustrating for someone who's not like that, especially if they were starting to think more seriously down the line.
if something like that doesn't work, a casual ex would be great or a friendly type!